Article
Spectroscopic and Theoretical Investigation of a Complex with an [O═FeIV–O–FeIV═O] Core Related to Methane Monooxygenase Intermediate Q
† Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
‡ Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota 55455, United States
J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/ja411376u
Publication Date (Web): December 31, 2013
Copyright © 2013 American Chemical Society
Abstract
Previous efforts to model the diiron(IV) intermediate Q of soluble methane monooxygenase have led to the synthesis of a diiron(IV) TPA complex, 2, with an O=FeIV–O–FeIV–OH core that has two ferromagnetically coupled Sloc = 1 sites. Addition of base to 2 at −85 °C elicits its conjugate base 6 with a novel O═FeIV–O–FeIV═O core. In frozen solution, 6 exists in two forms, 6a and 6b, that we have characterized extensively using Mössbauer and parallel mode EPR spectroscopy. The conversion between 2 and 6 is quantitative, but the relative proportions of 6a and 6b are solvent dependent. 6a has two equivalent high-spin (Sloc = 2) sites, which are antiferromagnetically coupled; its quadrupole splitting (0.52 mm/s) and isomer shift (0.14 mm/s) match those of intermediate Q. DFT calculations suggest that 6aassumes an anti conformation with a dihedral O═Fe–Fe═O angle of 180°. Mössbauer and EPR analyses show that 6b is a diiron(IV) complex with ferromagnetically coupled Sloc = 1 and Sloc= 2 sites to give total spin St = 3. Analysis of the zero-field splittings and magnetic hyperfine tensors suggests that the dihedral O═Fe–Fe═O angle of 6b is 
90°. DFT calculations indicate that this angle is enforced by hydrogen bonding to both terminal oxo groups from a shared water molecule. The water molecule preorganizes 6b, facilitating protonation of one oxo group to regenerate 2, a protonation step difficult to achieve for mononuclear FeIV═O complexes. Complex 6 represents an intriguing addition to the handful of diiron(IV) complexes that have been characterized.
90°. DFT calculations indicate that this angle is enforced by hydrogen bonding to both terminal oxo groups from a shared water molecule. The water molecule preorganizes 6b, facilitating protonation of one oxo group to regenerate 2, a protonation step difficult to achieve for mononuclear FeIV═O complexes. Complex 6 represents an intriguing addition to the handful of diiron(IV) complexes that have been characterized.
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